External storage/harddrive newbie question...

I have a Rocstor 1tb that Time Machine on my iMac desktop backs up to regularly. I would like to have another layer of security that would not need to be plugged into my home's electrical system, say in case of lightening strikes. Some folks seem to rely on various Cloud services, but the Apple folks say the data may be corrupted when you need to download it. I bought an AirPort Time Capsule, but I think it may be much more than I need…especially since I do not use wireless at home...I have read over the topic "Which external hard drive should I buy" and gotten some good ideas, but I have one newbie question…say you have another harddrive or storage device, will these store your photos and music without being plugged in? I mean, you'd plug them in to do backup, but from what I've read, it sounds like you can unplug them and they will retain that data. Thanks in advance, jay